The present invention concerns a novel system and process for separating impurities from water using reverse osmosis membranes.
The process of reverse osmosis is currently in widespread use for the treatment of water. Its value is derived from the ability of a semipermeable membrane to preferentially reject the passage of most salts, a high percentage of organic contaminants, and nearly all particulate matter. The process, as it is typically designed and operated, has limitations though, in that both the dissolved salt and organic passage is too great for the process to be used as a stand-alone treatment, and the seals separating the raw and treated water sides of the membrane are not sufficiently reliable to assure consistently high rejection of particulates.
Additionally, it is customary, and in many cases imperative, to pretreat the water supply ahead of reverse osmosis to avoid fouling of membrane surfaces. Doing so in many cases limits the performance of the unit because the required pretreatment can reduce the ability of the membranes to reject contaminants.
In the majority of raw water supplies, calcium and alkalinity levels are sufficiently high that direct treatment by reverse osmosis would cause precipitation of calcium carbonate on membrane surfaces, reducing productivity. To avoid precipitation, pretreatment by softening, or by acid addition, is practiced. Both processes reduce the effectiveness of the reverse osmosis membrane. When softening is employed, the divalent ions of calcium and magnesium are exchanged for the monovalent ion sodium. Sodium is not as well rejected by the membrane, thus the treated water salt level is increased and the cost of further removal of dissolved solids downstream of the reverse osmosis system is increased. If acid is added to reduce alkalinity, the alkalinity is converted to carbonic acid, which passes freely through the membrane and thus also increases the cost of downstream treatment.
It is an object of the present invention to provide a system and process which significantly alleviates the aforementioned difficulties.
Another object of the present invention is to provide a system and process for purifying water that is relatively low in cost and simple to produce and use.
A further object of the present invention is to provide a water purification system and process, using reverse osmosis techniques, that is reliable in operation and achieves high particle rejection.
A still further object of the present invention is to provide a reverse osmosis type water purification system and process which enables the operator to tailor the system and process to performance improvement.
Other objects and advantages of the present invention will become apparent as the description proceeds.